Electronic weighing apparatus utilizing surface acoustic waves

ABSTRACT

A weighing apparatus which includes a SAW oscillator ( 120 b,  122 b) and a “push oscillator” ( 216, 218, 220 ) to force the SAW oscillator into a desired mode of operation. The device adjusts to temperature by using a temperature sensing SAW transducer assembly ( 124, 126 ). Long term temperature stability of the SAW frequency oscillator is achieved by periodic calibration with a thermistor ( 224 ) signal.

[0001] This application is related to allowed co-owned application Ser.No. 09/327,707 filed Jun. 9, 1999, Ser. No. 08/729,752 filed Oct. 7,1996, now U.S. Pat. No. 5,910,647, and Ser. No. 08/489,365 filed Jun.12, 1995, now U.S. Pat. No. 5,663,531, the complete disclosures of whichare hereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to electronic weighing devices. Moreparticularly, the invention relates to an electronic weighing devicewhich employs surface acoustic waves to measure weight.

[0004] 2. State of the Art

[0005] Precision electronic weighing devices are widely known in the artand there are many different technologies utilized in these electronicweighing devices. Laboratory scales or “balances” typically have acapacity of about 1,200 grams and a resolution of about 0.1 gram,although scales with the same resolution and a range of 30,000 grams areavailable. The accuracy of these scales is achieved through the use of atechnology known as magnetic force restoration. Generally, magneticforce restoration involves the use of an electromagnet to oppose theweight on the scale platform. The greater the weight on the platform,the greater the electrical current needed to maintain the weight. Whilethese scales are very accurate (up to one part in 120,000), they areexpensive and very sensitive to ambient temperature. In addition, theirrange is relatively limited.

[0006] Most all other electronic weighing devices use load celltechnology. In load cell scales, the applied weight bends an elasticmember which has strain gauges bonded to its surface. The strain gaugeis a fine wire which undergoes a change in electrical resistance when itis either stretched or compressed. A measurement of this change inresistance yields a measure of the applied weight. Load cell scales areused in non-critical weighing operations and usually have a resolutionof about one part in 3,000. The maximum resolution available in a loadcell scale is about one part in 10,000 which is insufficient for manycritical weighing operations. However, load cell scales can have acapacity of several thousand pounds.

[0007] While there have been many improvements in electronic weighingapparatus, there remains a current need for electronic weighingapparatus which have enhanced accuracy, expanded range, and low cost.

[0008] The previously incorporated applications disclose an electronicweighing apparatus having a base which supports a cantilevered elasticmember upon which a load platform is mounted. The free end of theelastic member is provided with a first piezoelectric transducer and asecond piezoelectric transducer is supported by the base. Eachtransducer includes a substantially rectangular piezoelectric substrateand a pair of electrodes imprinted on the substrate at one end thereof,with one pair of electrodes acting as a transmitter and the other pairof electrodes acting as a receiver. The transducers are arranged withtheir substrates substantially parallel to each other with a small gapbetween them and with their respective electrodes in relatively oppositepositions. The receiver electrodes of the second transducer are coupledto the input of an amplifier and the output of the amplifier is coupledto the transmitter electrodes of the first transducer. The transducersform a “delay line” and the resulting circuit of the delay line and theamplifier is a positive feedback loop, i.e. a natural oscillator. Moreparticularly, the output of the amplifier causes the first transducer toemit a surface acoustic wave (“SAW”) which propagates along the surfaceof the first transducer substrate away from its electrodes. Thepropagating waves in the first transducer induce an oscillating electricfield in the substrate which in turn induces similar SAW waves on thesurface of the second transducer substrate which propagate in the samedirection along the surface of the second transducer substrate towardthe electrodes of the second transducer. The induced waves in the secondtransducer cause it to produce an alternating voltage which is suppliedby the electrodes of the second transducer to the amplifier input. Thecircuit acts as a natural oscillator, with the output of the amplifierhaving a particular frequency which depends on the physicalcharacteristics of the transducers and their distance from each other,as well as the distance between the respective electrodes of thetransducers.

[0009] When a load is applied to the load platform, the free end of thecantilevered elastic member moves and causes the first transducer tomove relative to the second transducer. The movement of the firsttransducer relative to the second transducer causes a change in thefrequency at the output of the amplifier. The movement of the elasticmember is proportional to the weight of the applied load and thefrequency and/or change in frequency at the output of the amplifier canbe calibrated to the displacement of the elastic member. The frequencyresponse of the delay line is represented by a series of lobes. Eachmode of oscillation is defined as a frequency where the sum of thephases in the oscillator is an integer multiple of 27. Thus, as thefrequency of the oscillator changes, the modes of oscillation movethrough the frequency response curve and are separated from each otherby a phase shift of 2X. The mode at which the oscillator will mostnaturally oscillate is the one having the least loss. The transducersare arranged such that their displacement over the weight range of theweighing apparatus causes the oscillator to oscillate in more than onemode. Therefore, the change in frequency of the oscillator as plottedagainst displacement of the transducers is a periodic function. Thereare several different ways of determining the cycle of the periodicfunction so that the exact displacement of the elastic member may bedetermined.

[0010] It is generally known in the art of SAW technology that thefrequency range in which the losses are the lowest is not necessarilythe frequency range in which the oscillator exhibits the best phaselinearity. From the teachings of the previously incorporatedapplications, those skilled in the art will appreciate that in a SAWdisplacement transducer such as disclosed in the previously incorporatedapplications, better phase linearity provides a more linear relationshipbetween frequency and displacement. In the case of a weighing apparatususing a SAW displacement transducer as described in the previouslyincorporated applications, better phase linearity will result in a morelinear relationship between weight and frequency.

[0011] It is known in the art of SAW oscillators that changing thetopology of the oscillator transmitter and receiver can cause a broaderbandwidth of the delay line and that a broader bandwidth results inbetter phase linearity. It is also known that using a smaller frequencyrange provides better linearity and that a smaller frequency range canbe obtained with a longer delay line. Although these known methods canincrease phase linearity in a SAW oscillator, the frequency range inwhich the best linearity is achieved for a particular oscillator isstill not necessarily the range with the lowest losses.

[0012] From the foregoing, those skilled in the art will appreciate thatin order to enhance the accuracy of a SAW displacement transducer suchas that used in a weighing device, it would be desirable to cause theSAW oscillator to oscillate in the range having the best phaselinearity.

[0013] As disclosed in the previously incorporated applications,weighing accuracy is affected by temperature. The previouslyincorporated applications disclose a SAW temperature oscillator having atransmitter and receiver on the same substrate. The temperaturesensitivity of the load cell disclosed in the previously incorporatedapplications is approximately 500 ppm of the weight reading per 1° C.temperature change. Accuracy of 100 ppm of the weight reading can beachieved if temperature is measured to within 0.2° C. which represents ashift of about 1 kHz of the SAW temperature sensor. This shift is easyto measure in the short term. The resolution of the SAW temperaturesensor is on the order of 0.001° C. However, the long term stability ofthe SAW temperature sensor can drift more than 1 kHz due to many factorsincluding humidity

[0014] It will also be appreciated that temperature changes can makedetermination of mode of oscillation more difficult. In the previouslyincorporated applications, mode determination was determined byswitching the phase ±π and noting the frequency change. However, factorssuch as temperature and non-linearity can make the frequency change verysmall thereby making the mode determination unreliable

SUMMARY OF THE INVENTION

[0015] It is therefore an object of the invention to provide anelectronic weighing apparatus which is accurate.

[0016] It is also an object of the invention to provide an electronicweighing apparatus which uses surface acoustic waves and is accurateover a broad range of weights.

[0017] It is another object of the invention to provide an electronicweighing apparatus which is compact and easy to construct.

[0018] It is a further object of the invention to provide an electronicweighing apparatus which is inexpensive to manufacture.

[0019] It is another object of the invention to provide an electronicweighing apparatus which utilizes surface acoustic waves and which hasenhanced phase linearity.

[0020] It is still another object of the invention to provide anelectronic weighing apparatus which utilizes surface acoustic waves andwhich is oscillates in the mode of best phase linearity.

[0021] It is yet another object of the invention to provide anelectronic weighing apparatus which utilizes surface acoustic waves andhas long term temperature stability as well as short term temperaturestability.

[0022] In accord with these objects which will be discussed in detailbelow, the improved weighing apparatus of the present invention includesa base which supports a cantilevered elastic member upon which a loadplatform is mounted. The interior of the elastic member is hollowed andis provided with first and second piezoelectric transducers which aremounted on respective opposed posts. Each transducer includes asubstantially rectangular piezoelectric substrate and a pair ofelectrodes imprinted on the substrate at one end thereof, with one pairof electrodes acting as a transmitter and the other pair of electrodesacting as a receiver. The transducers are arranged with their substratessubstantially parallel to each other with a small gap between them andwith their respective electrodes in relatively opposite positions. Thereceiver electrodes of the second transducer are coupled to the input ofan amplifier and the output of the amplifier is coupled to thetransmitter electrodes of the first transducer. The transducers form a“delay line” and the resulting circuit of the delay line and theamplifier is a positive feedback loop, i.e. a natural oscillator. Moreparticularly, the output of the amplifier causes the first transducer toemit a surface acoustic wave (“SAW”) which propagates along the surfaceof the first transducer substrate away from its electrodes. Thepropagating waves in the first transducer induce an oscillating electricfield in the substrate which in turn induces similar SAW waves on thesurface of the second transducer substrate which propagate in the samedirection along the surface of the second transducer substrate towardthe electrodes of the second transducer. The induced waves in the secondtransducer cause it to produce an alternating voltage which is suppliedby the electrodes of the second transducer to the amplifier input. Thecircuit acts as a natural oscillator, with the output of the amplifierhaving a particular frequency which depends on the physicalcharacteristics of the transducers and their distance from each other,as well as the distance between the respective electrodes of thetransducers.

[0023] According to the invention, when a load is applied to the loadplatform, the cantilevered elastic member bends and causes the firsttransducer to move relative to the second transducer. The movement ofthe first transducer relative to the second transducer causes a changein the frequency at the output of the amplifier. The bending movement ofthe elastic member is proportional to the weight of the applied load andthe frequency and/or change in frequency at the output of the amplifiercan be calibrated to the displacement of the elastic member.

[0024] According to one aspect of the invention, a “push oscillator” iscoupled to the delay line for injecting a strong RF signal at afrequency in the middle of the oscillation mode which exhibits the bestphase linearity. The frequency of the “push oscillator” is determinedexperimentally when the scale is calibrated. The RF signal is injectedperiodically in short bursts.

[0025] According to a second aspect of the invention, the “pushoscillator” frequency is generated by mixing the temperature oscillatorwith an adjustable fixed frequency oscillator. This immunizes the “pushoscillator” from the affects of temperature.

[0026] According to a third aspect of the invention, a thermistor isprovided for long term temperature stability. The SAW temperature sensoris periodically calibrated to the thermistor.

[0027] According to a fourth aspect of the invention, the SAWoscillators are not hermetically sealed and the SAW temperature sensoris used to correct the displacement sensor for changes in humidity.

[0028] According to a fifth aspect, a single amplifier is used to powertwo SAW sensors (e.g. a weight sensor and a temperature sensor) whichoscillate at different frequencies.

[0029] According to a sixth aspect of the invention, a crystaloscillator is used to compensate the push oscillator for temperaturechanges.

[0030] According to a seventh aspect of the invention, two SAW weightsensors are provided where each oscillates at a different frequency.

[0031] Additional objects and advantages of the invention will becomeapparent to those skilled in the art upon reference to the detaileddescription taken in conjunction with the provided figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 is a schematic side elevation view of an exemplaryembodiment of the invention;

[0033]FIG. 1a is an enlarged schematic plan view of a first transducer;

[0034]FIG. 1b is an enlarged schematic plan view of a second transducer;

[0035]FIG. 2 is an enlarged schematic side elevation view of atransducer having anti-reflection structure according to the invention;

[0036]FIG. 3 is an enlarged schematic side elevation view of a pair oftransducers according to the invention;

[0037]FIGS. 4 and 5 are graphs of a portion of a frequency responsecurve for a delay line according to the invention showing modes ofoscillation and phase shifting according to the invention;

[0038]FIG. 6 is a simplified schematic diagram of circuits used in theweighing device according to the invention;

[0039]FIG. 7 is a simplified schematic diagram of a circuit wherein twoSAW sensors are powered by a single amplifier;

[0040]FIG. 8 is a simplified schematic diagram of a circuit used tocompensate the push oscillator for temperature;

[0041]FIG. 9 illustrates the frequency shift of two displacement sensorshaving different frequencies; and

[0042]FIG. 10 illustrates the frequency shift of two displacementsensors having different frequencies and arranged in a differentialmanner.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] Referring now to FIGS. 1, 1a, and 1 b, an electronic weighingapparatus 10 according to the invention includes a base 12 whichsupports a cantilevered elastic member 14 having a cut-out 15, and uponwhich a load platform 16 is mounted. The cut-out 15 is provided with twoopposed posts 17, 19 upon which are respectively mounted a firstpiezoelectric transducer 20 and a second piezoelectric transducer 22.The posts 17, 19 serve to locate the transducers 20, 22 at the center ofthe elastic member 14 and to mechanically couple the transducers toopposite ends of the elastic member 14.

[0044] The first transducer 20 includes a substantially rectangularpiezoelectric substrate 20 a and a pair of electrodes 20 b imprinted onthe substrate at the upper end thereof. The second transducer 22includes a substantially rectangular piezoelectric substrate 22 a and apair of electrodes 22 b imprinted on the substrate at the lower endthereof. The substrates are preferably made of Lithium Niobate. Thetransducers are arranged with their substrates substantially parallel toeach other with a small gap “g” between them. The electrodes 22 b of thesecond transducer 22 are coupled to the input of an amplifier (notshown) powered by a power source (not shown) and the output of theamplifier is coupled to the electrodes 20 b of the first transducer 20.The circuit arrangement is the same as shown in the previouslyincorporated application Ser. No. 08/489,365, previously incorporatedherein by reference. The resulting circuit is a positive feedback loopnatural oscillator, a “delay line”. The output of the amplifiergenerates an alternating voltage in the electrodes 20 b of the firsttransducer 20 which generates a surface acoustic wave (“SAW”) 26 whichpropagates along the surface of the first transducer substrate 20 a awayfrom its electrodes 20 b. Since the substrate 20 a of the firsttransducer 20 is relatively close to the substrate 22 a of the secondtransducer 22, an oscillating electric field which is induced as aresult of the SAW waves 26 in the piezoelectric substrate 20 a is ableto in turn induce similar SAW waves 28 on the surface of the secondtransducer substrate 22 a which propagate in the same direction alongthe surface of the second transducer substrate toward the electrodes 22b of the second transducer 22. The induced waves 28 in the secondtransducer 22 cause the electrode 22 b of the second transducer 22 toproduce an alternating voltage which is provided to the input of theamplifier. As long as the gain of the amplifier 24 is larger than theloss of the system, the circuit acts as a natural oscillator with theoutput of the amplifier having a particular frequency which depends onthe physical characteristics of the transducers and their distance fromeach other, as well as the distance between the respective electrodes ofthe transducers. In particular, the frequency of the oscillator isdirectly related to the time it takes for the SAW to propagate from theelectrodes 20 b to the electrodes 22 b.

[0045] According to presently preferred embodiments of the invention,described in more detail below, the SAW 26 has a wavelength ofapproximately 100-200 microns at 20-50 MHz. In order to limit loss inthe system, the gap “g” between the substrates of the first and secondtransducers is kept small. In one preferred embodiment described below,the gap is 10-20 microns. With such a gap, an oscillating system cantypically be generated if the amplifier 24 has a gain of at leastapproximately 25 dB. It will be appreciated that when a load (not shown)is applied to the load platform 16, the free end of the cantileveredelastic member 14 moves down and causes the second transducer 22 to moverelative to the first transducer 20. In particular, it causes theelectrodes 22 b of the second transducer 22 to move away from theelectrodes 20 b of the first transducer 20. This results in alengthening of the “delay line”. The lengthening of the delay linecauses an decrease in the frequency at the output of the amplifier. Thedisplacement of the elastic member is proportional to the weight of theapplied load and the frequency or decrease in frequency at the output ofthe amplifier can be calibrated to the distance moved by the elasticmember.

[0046] It will be appreciated that locating the transducers at thecenter of the elastic member compensates for any torque on the memberwhich would exhibit itself at the free end of the member. This resultsin an improved accuracy as compared to the weighing instrument disclosedin U.S. Pat. No. 5,663,531. Depending on the application (e.g. maximumload to be weighed), the elastic member is made of aluminum or steel.The presently preferred elastic member exhibits a maximum displacementof 0.1 to 0.2 mm at maximum load.

[0047] Reflected waves may occur on both piezo substrates. Reflectedwaves interfere with the received signal. The interference causes anincrease in non-linearity. FIGS. 2 and 3 show presently preferredanti-reflection structures according to the invention.

[0048] Turning now to FIGS. 2 and 3, presently preferred transducers120, 122 are shown. FIG. 2 illustrates the features of transducer 120which is substantially identical to transducer 122. FIG. 3 illustratesthe transducers mounted on the posts 17, 19 of the elastic member 14 ofFIG. 1. As shown in FIG. 2, the transducer 120 includes a lithiumniobate substrate 120 a with electrodes 120 b imprinted thereon. Theends 120 c, 120 d of the substrate are tapered and polyurethane dampers121 a, 121 b are placed at the ends to minimize reflection of the SAWwaves.

[0049] As mentioned above and in the previously incorporatedapplication, the delay lines according to the invention may oscillate inmore than one mode and within each mode, the gain will vary as thefrequency changes. Referring now to FIGS. 4 and 5, in the idle state,with no weight applied to the scale, the delay line will oscillate at afrequency “f” which is shown in FIG. 4 as the point having the most gain(least loss). The optimal gain area of the graph of FIG. 4 is shown inthe shaded area surrounding f and represents a range of ±100 Khz, forexample. This area is considered optimal because it is the area of leastloss. However, it is not necessarily the area of best phase linearity.After experimenting, it may be discovered that oscillation in adifferent mode, e.g. the shaded area of FIG. 5, will produce betterphase linearity. According to one aspect of the invention, theoscillator is forced to oscillate in the mode of best phase linearity byinjecting a strong RF signal having a frequency at the midpoint of thedesired mode of oscillation. The RF signal is injected by a “pushoscillator” coupled to the SAW wave receiver as described in more detailbelow with reference to FIG. 6. According to the presently preferredembodiment, the RF signal has a strength of approximately 100 mv ascompared to the SAW oscillator's strength of approximately 10 mv. The RFsignal is preferably injected for a short time (as short as 0.01seconds) before each weight measurement.

[0050] As mentioned above, and described in detail in the previouslyincorporated applications, the effects of temperature can be furthercorrected by providing a separate SAW temperature sensor on the samesubstrate as one of the displacement transducers. According to thepresently preferred embodiment, the SAW displacement oscillator operatesat 55 MHz and the SAW temperature oscillator operates at 57 MHz.According to another aspect of the invention, described in more detailbelow with reference to FIG. 6, the temperature oscillator is used inconjunction with an adjustable 2 MHz oscillator and a mixer to producethe “push oscillator” frequency and automatically adjust the “pushoscillator” frequency for temperature changes.

[0051] Turning now to FIG. 6, an exemplary circuit 200 according to theinvention includes the displacement SAW transducer formed by thetransmitter 122 b on the substrate 122 and the receiver 120 b on thesubstrate 120 coupled to each other by the amplifier 202. In addition,the circuit includes a temperature SAW transducer formed by thetransmitter 124 and receiver 126 on the substrate 122 coupled to eachother by the amplifier 204. The output of amplifier 202 is a frequencyFw which varies according to displacement of the substrates relative toeach other, which is an indication of weight when the transducers arearranged as shown in FIG. 1. According to the presently preferredembodiment, the frequency Fw is nominally 54 MHz. Fw will also varyaccording to temperature. The output of amplifier 204 is a frequency Ftwhich varies only according to temperature and humidity and which isnominally 57 MHz. The frequencies Fw and Ft are mixed (subtracted) atthe mixer 206 to produce a nominal frequency of 3 MHz which variesaccording to weight and which is temperature compensated. The outputfrequency of the mixer 206 is input to a microprocessor 208 whichcalculates weight as described in the previously incorporatedapplications and displays the weight on display 210. According to thepresently preferred embodiment, the output Ft of amplifier 204 is alsomixed via mixer 212 with a 54 MHz signal from oscillator 214 to producea signal which is nominally 3 MHz and which varies only with temperatureand humidity. The signal Fw-Ft provides a temperature adjusted weightsignal which accounts for the affects of temperature on the SAWoscillators. It does not compensate for temperature effects on theYoungs modulus of the elastic member (14 in FIG. 1). The signal outputfrom mixer 212 is a pure temperature indicator and is used to adjust theweight calculation for the effects of temperature on the Youngs modulusof the elastic member.

[0052] According to one aspect of the present invention, a “pushoscillator” is formed from an adjustable oscillator 216, a mixer 218,and a modulator 220. The oscillator 216 has a nominal frequency of 2 MHzwhich is mixed via the mixer 218 with the output of amplifier 204 toproduce an output frequency Fi which is (Ft—approx. 2 MHz). Thisfrequency Fi is used to index the modulator 220 which produces the “pushoscillator” output to the input of amplifier 202. As shown in FIG. 6,the modulator 220 and the oscillator 216 are both coupled to themicroprocessor 208. The microprocessor 208 is programmed to periodicallyactivate the modulator 220 to inject the push frequency as describedabove. In addition, the microprocessor advantageously is utilized toadjust the oscillator 216 to determine the frequency of the “pushoscillator”. The oscillator 216 may be initially adjusted via a simplevariable resistor or variable capacitor. However, it is further adjustedby the microprocessor during operation of the scale. One of theseadvantages is that the microprocessor can adjust the oscillator 216 toproduce the phase shifting described in the previously incorporatedapplications. In addition, it can be used to produce much largerfrequency shifts than were possible in the previously incorporatedapplications. This results in more accurate determinations of whichweight range the scale is in. As described in the previouslyincorporated applications, the oscillator operated as a periodicfunction where the same frequencies were repeated over different weightranges. A phase shift of ±π was used to determine which weight range thescale was operating in. As the weight increased, the same phase shiftproduced a larger frequency shift (because of the increased length ofthe delay line) and the frequency shift could be used to determine theweight range. However, under some circumstances, the phase shiftresulted in a frequency shift which was too small to accuratelydetermine. The push oscillator of the present invention can be used toproduce ±4π phase shifts.

[0053] As mentioned above, the oscillator 216 is preferably initiallyadjusted with a variable resistor or variable capacitor to ensureoscillation on the mode of best phase linearity. Initial calibration isperformed as follows: Known weights are placed on the scale and thefrequency of the oscillator output is determined for different weightsand the modes of oscillation are noted. The push oscillator is tuned tooperate in one mode and experiments are conducted to measure linearity.The experiments are repeated for each mode. The push oscillator is thentuned to push to the mode of best linearity.

[0054] According to another aspect of the invention, a thermistor 224 iscoupled via an analog to digital converter 226 to the microprocessor208. The thermistor is used to provide long term calibration of the SAWtemperature transducer. At first calibration measurements are taken fromboth the SAW temperature sensor and the thermistor assuming that thetrue temperature is the thermistor reading. Measurements are taken overa range of 20 or 40° C. and the slope of the SAW temperature sensoroutput is calculated in HzI° C. This slope is nominally approximately5,000 Hz/° C. using an oscillator having a 57 MHz central frequency.Periodically, this calibration is repeated to account for the long terminstability of the SAW temperature sensor.

[0055] Although one of the earlier applications proposed hermeticallysealing sensors, it has been determined that the effects of humidity areaccurately accounted for with the SAW temperature sensor. Therefore, ithas been determined to be desirable to expose both the displacementtransducer and the temperature transducer to the temperature andhumidity of the ambient atmosphere.

[0056] As described above and in the previously incorporated relatedpatents, a scale according to the invention will typically use two ormore SAW devices. One may be a temperature sensor and the other a weightsensor. Normally, each has its own amplifier to form an oscillator. Thetemperature sensor is typically be used to compensate for temperatureand other changes that occur with the weight sensor. However, thetemperature sensor can not compensate for differences caused bydifferences in the amplifier and other devices in the oscillator.

[0057] Referring now to FIG. 7, it is possible to connect two SAWsensors in parallel to a single amplifier. As shown in FIG. 7, a weightsensor having transducers 300, 302 is coupled in parallel with atemperature sensor having transducers 304, 306 to a single amplifier308. Each sensor oscillates at a somewhat different center frequency,such as 50 Mhz and 60 Mhz, respectively. A push oscillator 310controlled by a processor control 312 is arranged to force theoscillator to either 50 Mhz or 60 Mhz. When it forces the oscillator to50 Mhz, and is then shut off, the first SAW sensor will control thefrequency of oscillation output at 314. When the push oscillator forcesa frequency of 60 Mhz and then shuts off, the second SAW sensor willcontrol the frequency of oscillation output at 314. As the sameamplifier is shared by both SAW sensors, errors caused by differences inthe amplifiers used by weight and temperature sensors will beautomatically compensated for.

[0058] As described above, a push oscillator can be used to force theSAW oscillator to operate in the preferred area of oscillation. However,as the ambient temperature changes, the preferred frequency ofoscillation can also change. It would therefore be advantageous tochange the frequency of the push oscillator as the temperature changes.One method of doing this is to measure the temperature, either with aSAW temperature sensor or some other temperature sensor and adjust thepush oscillator, if the push oscillator is a phase locked loop or someother controllable type.

[0059] Another method, illustrated in FIG. 8, is to use the SAWtemperature sensor 406, 404, 408 along with a fixed oscillator, such asa crystal oscillator 410. If the frequencies are properly chosen, andthe two outputs are mixed with a mixer 412, the resultant frequencyoutput at 414 can be used as the push oscillator for the SAW weightsensor. For example, with the SAW temperature sensor center frequency at54 Mhz, and the SAW weights sensor center frequency at 50 Mhz, the fixedcrystal oscillator should have a 4 Mhz frequency. When the temperatureoscillator and the crystal oscillator outputs are mixed in a mixer, theoutput is centered at 50 Mhz, but will vary with temperature.

[0060] As described above and in the previously incorporated patents, ina typical scale the SAW device(s) will be displaced by more than onewavelength. Thus, there will be certain weights which result inidentical frequencies of oscillation. A number of methods have beendescribed as to how to determine which “zone” the oscillator isoperating in. One method was to use a completely different “coarse”sensor, such as strain gages, or inductive or capacitive sensors. Thesegives a coarse indication of weight, and therefore determines which“zone” the SAW sensor was operating in.

[0061] According to the present invention two weight sensors areprovided, each oscillating at a different frequency. Although bothsensors will be displaced by more than one wavelength, they will differin wavelength. FIG. 9 illustrates the frequency change of two weightsensors operating at different frequencies of oscillation, one shown ina solid line and the other shown in a dashed line. Those skilled in theart will appreciate that at any given weight, the pair of frequencyvalues will be unique.

[0062] As described above and in the previously incorporated patents, adifferential pair of SAW weight sensors may be used to improveresolution. When weight increases on the scale platform, one sensor willhave an increased distance between the transmitter and receiver, and theother will have a decreased distance. This is best shown in FIGS. 9-11of previously incorporated U.S. Pat. No. 5,910,647. According to thepresent invention, this arrangement can also be used to locate the zoneof oscillation by using different frequencies for each sensor.

[0063] A new method according to the invention is to use differentfrequencies for each of two or more SAW weight sensors in a differentialformat. FIG. 10 illustrates the change in frequency of two differentialoscillators operating at different frequencies.

[0064] There have been described and illustrated herein severalembodiments of an electronic weighing apparatus utilizing surfaceacoustic waves. While particular embodiments of the invention have beendescribed, it is not intended that the invention be limited thereto, asit is intended that the invention be as broad in scope as the art willallow and that the specification be read likewise. Thus, whileparticular frequencies have been disclosed, it will be appreciated thatother frequencies could be utilized. Moreover, while particularconfigurations have been disclosed in reference to the location oftransmitting and receiving electrodes, it will be appreciated that therespective locations of transmitters and receivers could be reversed.Furthermore, while several different aspects of the invention have beendisclosed as solving various problems, it will be understood that thedifferent aspects of the invention may be used alone or in combinationwith each other in configurations other than those shown. It willtherefore be appreciated by those skilled in the art that yet othermodifications could be made to the provided invention without deviatingfrom its spirit and scope as so claimed.

1. An electronic weighing apparatus, comprising: a) a displaceableelastic member means for receiving a load and being displaced by theload such that the displacement of said elastic member means is relatedto the weight of the load; b) a first piezoelectric transducer having afirst substrate and one of a first surface acoustic wave (SAW)transmitter and a first SAW receiver, said first piezoelectrictransducer being coupled to said elastic member; c) a secondpiezoelectric transducer having a second substrate and the other of saidfirst SAW transmitter and said first SAW receiver, said secondpiezoelectric transducer being mounted in close proximity to said firstpiezoelectric transducer such that said displacement of said elasticmember causes a corresponding displacement of one of said first andsecond piezoelectric transducers relative to each other; d) a firstamplifier having an input and an output, said input of said firstamplifier being coupled to said first SAW receiver and said output ofsaid first amplifier being coupled to said first SAW transmitter suchthat said first piezoelectric transducer, said first amplifier, and saidsecond piezoelectric transducer form a first oscillator having a firstoutput frequency; e) processor means coupled to said output of saidfirst amplifier; and f) a push oscillator coupled to said first SAWreceiver for altering said first output frequency, wherein displacementof said elastic member means causes a displacement of one of said firstand second piezoelectric transducers relative to each other and therebychanges said first output frequency, and said first output frequency isused by said processor means to determine an indication of the weight ofthe load.
 2. An electronic weighing apparatus according to claim 1,further comprising: g) switching means for periodically activating saidpush oscillator.
 3. An electronic weighing apparatus according to claim1, further comprising: g) a second SAW receiver on one of said first andsecond substrates; h) a second SAW transmitter on the same substrate asthe second SAW receiver; i) a second amplifier having an input and anoutput, said input of said second amplifier being coupled to said secondSAW receiver and said output of said second amplifier being coupled tosaid second SAW transmitter, said second SAW amplifier output beingindicative of temperature.
 4. An electronic weighing apparatus accordingto claim 3, further comprising: j) an adjustable oscillator having anoutput; k) a mixer having a first input, a second input, and an output,said first input being coupled to said output of said adjustableoscillator, said second input being coupled to said output of saidsecond SAW amplifier, wherein said output of said mixer provides anindex frequency for said push oscillator.
 5. An electronic weighingapparatus according to claim 3, wherein: said push oscillator includes,i) an adjustable oscillator having an output, ii) a mixer having a firstinput, a second input, and an output, said first input being coupled tosaid output of said adjustable oscillator, said second input beingcoupled to said output of said second SAW amplifier, and iii) amodulator having an input and an input, said input of said modulatorbeing coupled to said mixer and said output of said modulator being theoutput of said push oscillator.
 6. An electronic weighing apparatusaccording to claim 3, further comprising: j) a thermistor having anoutput indicative of temperature; and k) comparison means coupled tosaid thermistor and said second SAW amplifier for comparing thetemperature indication of said thermistor with the temperatureindication of said second SAW amplifier.
 7. An electronic apparatus formeasuring displacement, comprising: a) a first piezoelectric transducerhaving a first substrate and one of a first surface acoustic wave (SAW)transmitter and a first SAW receiver; b) a second piezoelectrictransducer having a second substrate and the other of said first SAWtransmitter and said first SAW receiver, said second piezoelectrictransducer being mounted in close proximity to said first piezoelectrictransducer; c) a first amplifier having an input and an output, saidinput of said first amplifier being coupled to said first SAW receiverand said output of said first amplifier being coupled to said first SAWtransmitter such that said first piezoelectric transducer, said firstamplifier, and said second piezoelectric transducer form a firstoscillator having a first output frequency; d) processor means coupledto said output of said first amplifier; and e) a push oscillator coupledto said first SAW receiver for altering said first output frequency,wherein displacement of one of said first and second piezoelectrictransducers relative to each other and thereby changes said first outputfrequency, and said first output frequency is used by said processormeans to determine an indication of the magnitude of the displacement.8. An electronic apparatus according to claim 7, further comprising: f)switching means for periodically activating said push oscillator.
 9. Anelectronic weighing apparatus according to claim 7, further comprising:f) a second SAW receiver on one of said first and second substrates; g)a second SAW transmitter on the same substrate as the second SAWreceiver; h) a second amplifier having an input and an output, saidinput of said second amplifier being coupled, to said second SAWreceiver and said output of said second amplifier being coupled to saidsecond SAW transmitter, said second SAW amplifier output beingindicative of temperature.
 10. An electronic apparatus according toclaim 9, further comprising: i) an adjustable oscillator having anoutput; j) a mixer having a first input, a second input, and an output,said first input being coupled to said output of said adjustableoscillator, said second input being coupled to said output of saidsecond SAW amplifier, wherein said output of said mixer provides anindex frequency for said push oscillator.
 11. An electronic apparatusaccording to claim 9, wherein: said push oscillator includes, i) anadjustable oscillator having an output, ii) a mixer having a firstinput, a second input, and an output, said first input being coupled tosaid output of said adjustable oscillator, said second input beingcoupled to said output of said second SAW amplifier, and iii) amodulator having an input and an input, said input of said modulatorbeing coupled to said mixer and said output of said modulator being theoutput of said push oscillator.
 12. An electronic weighing apparatusaccording to claim 9, further comprising: i) a thermistor having anoutput indicative of temperature; and j) comparison means coupled tosaid thermistor and said second SAW amplifier for comparing thetemperature indication of said thermistor with the temperatureindication of said second SAW amplifier.
 13. A method for improving thephase linearity of a SAW wave oscillator in an electronic weighingapparatus having a first amplifier having an input coupled to a firstSAW receiver and an output coupled to a first SAW transmitter, saidmethod comprising: periodically injecting an RF signal into the firstSAW receiver in order to change the mode of oscillation to the modehaving the best phase linearity.
 14. A method according to claim 13,wherein: the RF signal is approximately ten times stronger than thesignal produced by the first amplifier.
 15. A method according to claim13, wherein: the RF signal is injected for approximately 0.01 secondevery time a weight measurement is made.
 16. A method according to claim13, wherein the weighing apparatus has a second amplifier having aninput coupled to a second SAW receiver and an output coupled to a secondSAW transmitter with the output of the second amplifier being indicativeof temperature, said method further comprising: generating the RF signalby mixing the output of the second amplifier with the output of atunable oscillator.
 17. A method according to claim 16, furthercomprising: calibrating the RF signal by tuning the tunable oscillator.18. A method according to claim 16, further comprising: calibrating theoutput of the second amplifier to the output of a thermistor.
 19. Amethod for improving the phase linearity of a SAW wave oscillator in anelectronic apparatus for measuring displacement having a first amplifierhaving an input coupled to a first SAW receiver and an output coupled toa first SAW transmitter, said method comprising: periodically injectingan RF signal into the first SAW receiver in order to change the mode ofoscillation to the mode having the best phase linearity.
 20. A methodaccording to claim 19, wherein: the RF signal is approximately ten timesstronger than the signal produced by the first amplifier.
 21. A methodaccording to claim 20, wherein: the RF signal is injected briefly everytime a weight measurement is made.
 22. A method according to claim 19,wherein the apparatus has a second amplifier having an input coupled toa second SAW receiver and an output coupled to a second SAW transmitterwith the output of the second amplifier being indicative of temperature,said method further comprising: generating the RF signal by mixing theoutput of the second amplifier with the output of a tunable oscillator.23. A method according to claim 22, further comprising: calibrating theRF signal by tuning the tunable oscillator.
 24. A method according toclaim 22, further comprising: calibrating the output of the secondamplifier to the output of a thermistor.
 25. A weighing apparatus,comprising: a) a first SAW sensor including a first SAW transmitter anda first SAW receiver; b) a second SAW sensor including a second SAWtransmitter and a second SAW receiver; c) a displaceable member, atleast one of said first and second SAW sensors being coupled to saiddisplaceable member such that displacement of said displaceable memberresults in displacement of one of said transmitter and receiver relativeto the other, d) a amplifier having an input and an output, both of saidSAW sensors being coupled in parallel to said amplifier, wherein saidfirst SAW sensor is arranged to oscillate at a first frequency and saidsecond SAW sensor is arranged to oscillate at a second frequencydifferent from said first frequency.
 26. A weighing apparatus accordingto claim 25, further comprising: e) a controllable oscillator coupled tothe output of said amplifier.
 27. A push oscillator for use with a firstSAW oscillator to force the first SAW oscillator into a desired mode ofoperation: a) a temperature sensing oscillator having a first output; b)a fixed frequency oscillator having a second output; and c) a mixerhaving two inputs and a third output, wherein said first and secondoutputs are coupled to respective of said two inputs and said thirdoutput forms the output if said push oscillator.
 28. A push oscillatoraccording to claim 27, wherein: said fixed frequency oscillator is acrystal oscillator.
 29. A push oscillator according to claim 27,wherein: said temperature sensing oscillator is a second SAW oscillator.30. A weighing apparatus, comprising: a) a displaceable member; b) afirst SAW oscillator coupled to said displaceable member such thatdisplacement of said displaceable member causes a change in thefrequency of said first SAW oscillator; c) a second SAW oscillatorcoupled to said displaceable member such that displacement of saiddisplaceable member causes a change in the frequency of said second SAWoscillator; wherein the frequency of said first SAW oscillator and thefrequency of said second SAW oscillator are different.
 31. A weighingapparatus according to claim 30, wherein: said first SAW oscillator andsaid second SAW oscillator are arranged such that what said first SAWoscillator frequency increases due to displacement of said displaceablemember, said second SAW oscillator frequency decreases.